NXP Semiconductors
UM10694
SSL2109ADB1107 - 230 V/7 W GU10 high PF isolated LED driver
UM10694
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© NXP B.V. 2013. All rights reserved.
User manual
Rev. 1 — 9 April 2013
7 of 21
4. Functional description
4.1 Input filtering
A differential mode
π
filter is implemented to reduce differential mode noise originating
from the High-frequency (HF) switching currents/voltages in the primary of the converter
from returning to the mains. This noise must be kept to a minimum by:
•
Small primary current loop (C2 to pin 5 of T1, pin 3 of T1 to drain of external
MOSFET Q5, ground return from sense resistors R1, R24, R26 to C2 ground)
•
Snubber components D3, R17, C8 closely parallel to pins 5 and 3 of T1
•
Track length from pin 3 of T1 to drain of external MOSFET to be small in order to
reduce capacitance to surroundings
For this 7 W application, C2 must be chosen sufficiently large to act as a buffer for the HF
current flowing in the primary but also small enough to maintain a high PF where a value
of 100 nF for C2 is a good compromise.
4.2 PF adjustment
The divider network consisting of R5, R6, R7, R9, R10, D4, C10 and C11 modulates the
base of Q7 according to the rectified mains. Consequently, a modulating current set by
R11 flows through R8 and the sense current through R1, R24 and R26 is also
modulated. Depending on the divider resistor values, the HF current through the primary
has a current with a rectified mains modulation resulting in a high PF.
The PF can be increased/decreased by decreasing/increasing R5 and R6 and a
compromise can be reached between PF and line regulation.
4.3 RCD clamp
Leakage inductance of the transformer is in series with primary inductance and the drain
of MOSFET Q5. This leakage inductance together with the parasitic capacitance across
the drain source of Q5 causes HF ringing when Q5 switches off. If the leakage is not
minimized the voltage at the drain reaches high voltages levels which can damage Q5 if
voltage levels exceed the maximum allowable level.
An RCD clamp (R17, C8 and D3) prevents the voltage on the drain of Q5 exceeding the
maximum drain voltage when Q5 switches off.
4.4 Supply voltage
The supply voltage of the SSL2109A is achieved using an auxiliary winding. As further
advantages, the auxiliary winding provides, via a resistor R25 on its underwinding, both a
simple solution for improved load regulation using R12 and open-circuit protection (OCP)
via the NTC pin using R13.
4.4.1 Open-circuit protection
If no LED string is connected to the converter output, the voltage can quickly exceed the
maximum rating of the output capacitors C3 and C13.
The underwinding of the auxiliary is connected to resistor R25 to prevent this. The
voltage across R25 decreases as the output voltage increases. Consequently, the
voltage at the NTC pin decreases once the current through R13 exceeds the internal
current of the NTC pin. When the NTC voltage drops below 0.35 V then the SSL2109A
switches off.
